The broad objective of the proposed research is to document what infants know about their social world, when they know it, and how they come to distinguish social from nonsocial events, Three issues provide the immediate focus of the proposal. The first concerns the claim that infants of 2-3 months must understand the difference between persons and objects, because they respond "socially" only to the former (i.e., by smiling and pre-vocal communication). Three experiments are designed to test competing explanations of these social responses i.e., that they are (1) fixed action patterns elicited by the dynamic properties of persons (i.e.,vocalization and elastic movements of the face), (2) behaviors that are reinforced by such dynamic properties, or 930 legitimate attempts to communicate with recognized social partners. the behaviors of infants will therefore be compared when interacting with contingently and noncontingently responsive figures of various kinds: (a) regular persons, (b) regular objects, (c) figures that are mixtures of the physical and dynamic attributes of persons and objects, and (d) persons who attend or fail to attend to the infant when responding. The second issue governing the proposal bears on the further claim that young infants are capable of categorizing persons and objects. A second series of experiments is proposed to determine whether this is indeed the case, and if so, what specific attributes infants might use to form these categories and whether they perceive the coorelational structure of each. Following habituation to a number of different persons (or objects), patterns of generalization and recovery will be examined to new persons and objects, and to other stimuli composed of varying combinations of person and object attributes. The final issue informing the research bears on a current developmental controversy: whether infants are socially precocious from an early age or become so only gradually. Accordingly, infants of 3, 7, and 10 months will be compared in the experiments cited above, Overall, these studies should provide more definitive evidence regarding the nature of social understanding in infancy, and, as such, should contribute to our knowledge of early social and cognitive development.